1. Field of the Invention
This invention relates to a so-called TTL (through-the-lens) type focus detecting apparatus for detecting the focus of an objective lens on the basis of a light passing through the objective lens.
2. Description of the Prior Art
Focus detecting apparatuses of this type are well known, and also known is an apparatus in which, for example, the pupil of an objective lens is divided and the focus state of the objective lens is detected from the relative positional relation between a plurality of images formed by the light beams from the areas of the divided pupil.
FIGS. 1A and 1B of the accompanying drawings schematically show the construction of such apparatus. In FIG. 1A, reference numeral 1 designates the photo-taking lens of a camera, not shown. In this apparatus, there are disposed, in succession from the side of the photo-taking lens 1, a field stop 3 having an opening 2, a field lens 4, a stop 6 having openings 5a and 5b, secondary imaging lenses 7a and 7b and sensor arrays 8a and 8b. The field stop 3 is disposed in the predetermined imaging plane of the photo-taking lens 1 so that the image in the opening 2 is re-imaged on the photoelectric element arrays 8a and 8b by the secondary imaging lenses 7a and 7b. The field lens 4 has a power for placing the exit pupil of the photo-taking lens 1 and the surface of the stop 6 substantially in an imaging relation. Accordingly, a light passed through the area 1a of the exit pupil of the photo-taking lens 1 enters the opening 5a of the stop 6 and is imaged on the sensor array 8a. Also, a light passed through the area 1b of the exit pupil of the photo-taking lens 1 enters the opening 5b of the stop 6 and is imaged on the sensor array 8b.
When the photo-taking lens 1 is in its in-focus state, the distributions of quantity of light of the images re-imaged on the two sensor arrays 8a and 8b are equal to each other on the light-receiving surfaces of the sensor arrays 8a and 8b. On the other hand, when the photo-taking lens 1 is in its non-in-focus state, the distributions of quantity of light of the images re-imaged on the sensor arrays 8a and 8b vary relative to each other in accordance with the defocus amount of the lens 1 and therefore, the focus state of the photo-taking lens 1 can be determined through the correlation between the two distributions of quantity of light on the basis of the output signals from the two sensor arrays 8a and 8b.
However, in the focus detecting apparatus of this type according to the prior art, when the object to be photographed is dark, a sufficient quantity of light cannot be directed onto the sensors, and this has sometimes caused malfunctioning or the impossibility of judging the in-focus.
In such a case, there would occur to mind a countermeasure that the object to be photographed be illuminated by auxiliary illuminating means provided outside the optical path of the photo-taking lens 1 to thereby direct a sufficient quantity of light onto the sensors, but the use of such illuminating means provided outside the optical path of the photo-taking lens 1 may result in creation of parallax, which in turn may lead to a problem that the auxiliary light cannot be well directed into a distant field of view.
On the other hand, an apparatus in which a light beam is projected onto an object to be photographed through a photo-taking lens and the reflected light from the object to be photographed is directed to a sensor through the photo-taking lens to thereby detect the focus state of the photo-taking lens is disclosed, for example, in U.S. Pat. No. 4,357,085 (issued on Nov. 2, 1982). FIG. 1B of the accompanying drawings shows the basic construction of such apparatus. As shown there, a light source 12 and the light-receiving surface of a sensor 13 are disposed in planes F1 and F2, respectively, optically equivalent to the film surface F of a camera relative to a photo-taking lens 11. A light beam emitted from the light source 12 passes through a stop 14 and is reflected by the mirror 15a of a reflecting member 15, and passes through the photo-taking lens 11 and is projected onto an object to be photographed. The reflected light from the object to be photographed passes through the photo-taking lens 11, and then is reflected by a mirror 15b and enters the sensor 13.
When the photo-taking lens 11 is imaging the object to be photographed at a point A on the film surface F, the light source 12 is imaged on the object to be photographed by the photo-taking lens 11 and the spots of received light on the sensor 13 are imaged at a position A2 optically equivalent to the position A1 of the light source 12. However, when the photo-taking lens 11 is in its front focus state or its rear focus state, the spots of received light on the sensor 13 are deviated in opposite directions from the position A2 and therefore, by detecting the positions of the spots of received light, it is possible to determine whether the photo-taking lens 11 is in its in-focus state.
However, in this apparatus, when the object to be photographed lies at a long distance, the quantity of light reflected from the object to be photographed and entering the sensor 13 is reduced and therefore, a signal of good S/N ratio cannot be obtained, and this has resulted in the disadvantage that determination of in-focus cannot be accomplished.